The volt (symbol: V) is the SI derived unit of electromotive force, commonly called "voltage". It is also the unit for the related but slightly different quantity electric potential difference (also called "electrostatic potential difference"). It is named in honor of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, possibly the first chemical battery.
The volt is defined as the value of the voltage across a conductor when a current of one ampere dissipates one watt of power in the conductor. It can be written in terms of SI base units as: m2 · kg · s−3 · A−1. It is also equal to one joule of energy per coulomb of charge, J/C.
Josephson junction definition
Since 1990 the volt has been maintained internationally for practical measurement using the Josephson effect, where a conventional value is used for the Josephson constant, fixed by the 18th General Conference on Weights and Measures as:
This is typically used with an array of several thousand or tens of thousands of junctions, excited by microwave signals between 10 and 80 GHz (depending on the array design).
The relationship KJ = 2e/h is apparently exact, with no correction terms required in a practical implementation.
Water flow analogy
In the water flow analogy sometimes used to explain electric circuits by comparing them to water-filled pipes, voltage difference is likened to water pressure difference–the difference determines how quickly the electrons will travel through the circuit. Current (in amperes), in the same analogy, is a measure of the volume of water that flows past a given point per unit time (volumetric flow rate). The flow rate is determined by the width of the pipe (analogous to electrical conductivity), and the pressure difference between the front end of the pipe and the exit (analogous to voltage). The analogy extends to power dissipation: the power given up by the water flow is equal to flow rate times pressure, just as the power dissipated in a resistor is equal to current times the voltage drop across the resistor (watts = amperes × volts).
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